Process and apparatus for separating mixtures of solid particles



May 16, 1961 J. N. J. LEEMAN PROCESS AND APPARATUS FOR SEPARATING MIXTURES OF SOLID PARTICLES Filed June 11, 1956 Unite State atent PROCESS AND APPARATUS FOR SEPARATING MIXTURES OF SOLID PARTICLES Jan N. J. Leeman, Heerlen, Netherlands, assignor to Stamicarbon N.V., Heerlen, Netherlands Filed June 11, 1956, Ser. No. 592,707

12 Claims. (Cl. 209-1725) The present invention relates to particle separation and more particularly to a process and an apparatus for separating mixtures of solid substances into fractions differing in specific gravity, by means of a separating suspension consisting of magnetizable particles suspended in water, in which the separated fractions are passed across screens on which the suspension adhering to the fractions is rinsed off, and the resulting dilute suspension is led to a magnetic separator in which are obtained a concentrated fraction of magnetizable particles which is returned to the separating device and a dilute fraction of non-magnetizable particles, which is led to the sprayers over the screens.

In the known processes of this type, the specific gravity of. the suspension recovered in the magnetic separator and returned to the separating device is higher than the specific gravity needed for the treatment in the separating device. This relationship is advantageous in that the desired specific gravity can be established in the separating device simply by adding water. No special thickeners for the suspension supplied are necessary and there is no need of circulating large amounts of suspension by pumping.

Heretofore, the dilute suspension coming from the rinsing screens is sent directly to the magnetic separator, without undergoing any secondary treatment. This suspension contains many non-magnetizable particles, a con siderable portion of which is enclosed by the magnetizable particles and returned to the separating device together with the magnetic fraction. The result is that the separating medium is contaminated by fine non-magnetizable particles. Consequently, the viscosity of the separating medium is raised which results in a reduction in the accuracy of the separation.

In the known processes the fine non-magnetizable particles are removed by leading the dilute fraction of nonmagnetizable particles obtained in the magnetic separatorto a thickener, where these particles settle and are discharged. The clarified liquid coming from this thickener is used as rinsing water on the rinsing screens. Such a thickener raises the cost of the separating system and requires much space.

Accordingly, it is an object of the present invention to provide a process of the type described in which the concentrated suspension returned to the separating device contains a minimum amount of non-magnetizable particles so as to maintain a high degree of accuracy of the separation.

Another object of the present invention is the provision of an apparatus for separating a mixture of solid substance into fractions differing in specific gravity by the use of a magnetite suspension or the like having improved means embodied therein for maintaining the concentrated suspension returned to the separating device substantially free from contamination by fine non-magnetizable particles.

Another object of the present invention is the provision of. a process of the type described in which the dilute ice fraction recovered from the magnetic separator contains so few non-magnetic particles that it can be fed directly to the sprayers over the rinsing screens.

A further object of the present invention is the provision of an apparatus of the type described having means for treating the suspension prior to its entry into the magnetic separator which enables the dilute fraction obtained therefrom to be fed directly to the rinsing screens without the necessity of being passed through a thickener.

Still another object of the present invention is the provision of an apparatus of the type described having means embodied therein which materially reduces the non-magnetizable particle contamination of the returned suspension over that of prior art apparatus and which eliminates the need for thickeners which have heretofore been required in prior art apparatus.

A still further object of the present invention is the provision of a novel procedure and apparatus of the type described which is capable of achieving a more accurate separation than heretofore achieved, such separation being attained with the use of apparatus simpler and less expensive than the prior art apparatus and through procedures which are simpler and easier to carry out than those heretofore known.

These and other objects of the present .invention will become more apparent during the course of the following detailed description and appended claims.

The invention may best be understood with reference to the accompanying drawings wherein illustrative embodiments are shown.

In the drawings:

Figure 1 is a diagrammatic view of a coal preparation plant embodying the principles of the present invention;

Figure 2 is a fragmentary diagrammatic view showing a modified form of the plant; and

Figure 3 is a fragmentary diagrammatic view showing another modified form of the plant.

Referring now more particularly to the drawings, there is shown in Figure 1, a separating device 1 in which raw coal is separated by means of a magnetite suspension into washed coal and shales. However, the invention is not restricted to the separation of raw coal, but can be uti lized for separating other materials, such as ores and the like, while suspensions of other magnetizable materials may be used, as for example, ferrosilicon and the like. The separating device may be a wash box operating on the float-and-sink principle, a cyclone washer or other suitable device.

The coal to be washed is introduced into the separating device 1 at A, and the separated coal and shales fractions leave the separating device at B and C, together with an amount of separating suspension. The fractions are passed across screens 2, where the suspension can trickle off the separated fractions. This suspension, which has the same specific gravity as the suspension in the separating device, is collected in a tank 3 and returned by a pump 4 into the separator 1 through a conduit 5.

Subsequently, the separated fractions are led across screens 6, where they are rinsed by means of sprays 7 with clarified water, in order to remove the adhering suspension, after which the washed coal and shales are discharged at D and E. These screens may be of any suitable construction, as for example, vibrating screens with a mesh of 0.5 mm.

The dilute suspension collected under the screens 6 is supplied tangentially, through a conduit 8, tothe bent screening deck of a screening device 9. This screening deck is preferably constructed in accordance with the disclosure of application Serial No. 475,251 filed December 14, 1954, by F. I. Fontein, issued as Patent No. 2,916,142 on December 8, 1959, and comprises a plurality of bars disposed perpendicular to the direction in which the suspension to be screened moves. The bars are spaced apart to provide slots having a width of the order of 0.4 mm. As is described in the aforesaid application, the material, exclusive of the suspension, which passes through the screen will consist only of particles with diameters smaller than about half the width of the slots, or, in the present case, smaller than 0.2 mm. Moreover, about 90 percent of the liquid supplied to the screen will pass therethrough along with the particles suspended in this liquid. As the particle size or" the magnetite particles is smaller than 100a, this means that 90 percent of the magnetite is caught in the collecting tank under the screen.

The overflow from screening device 9 contains the coarser non-magnetizable particles and 10 percent of the magnetite suspension. This fraction is supplied to a vibrating screen 11, having a mesh of 0.2 mm., on which clean water is sprayed, as by sprayers 12. In this way, the magnetite still clinging to this fraction is rinsed off here, after which it is collected in a tank 13. The nonmagnetizable coarse particles, which remain behind on the screen, are discharged at F and may, if desired, be subjected to a further treatment, as for example in a froth flotation apparatus, to remove the coal. However, it is also possible to separately treat the dilute sus' pensions; that is, those obtained in screening the washed coal and shale, in two screening devices as described above, so that at P pure fractions of coal and shales are directly discharged.

The dilute suspension collected in the collecting tanks 10 and 13 is fed into a magnetic separator 15 through a conduit 14. In this magnetic separator a concentrated magnetite suspension is recovered, which can have a specific gravity of about 2.1. A pump 16 is provided for delivering the concentrated magnetite suspension into the collecting tank 3 through a conduit 17. In passing to the tank, the suspension may be led through demagnetizing coil 18. If the separating device consists of one or more cyclones it is not necessary to demagnetize the suspension, however. Subsequently, the suspension is pumped to the separating device, together with the suspension from the draining screens 2 which feed into the collecting tank. It will be understood that it is possible to pump the magnetite suspension recovered by the magnetic separator 15 directly to the separating device, without interposing the collecting tank 3. In either case, the specific gravity of the suspension fed into the separating device will exceed the specific gravity desired for the treatment in the separating device, which may be, for example, 1.6. By introducing a proper amount of water into the separating device as at G, the desired specific gravity can be established in a simple manner. The water can also be supplied together with the coal, if desired, by rinsing the raw coal, with water on screens, in which. process a large portion of the accompanying fine particles is removed. Dewatering should then be carried out in such a way that the amount of water supplied into the separating device together with the wetted coal is sufiicient to eflect the desired compensation of the specific gravity. Of course, it is also possible to apply both measures, so that the raw coal is rinsed and afterwards as much water is supplied into the separating device as is necessary to establish the desired specific gravity.

In addition to the above, a dilute non-magnetic fraction is separated off in the magnetic separator. This fraction contains only the non-magnetizable particles smaller than 0.2 mm. The concentration is so low, however, that there is no need to remove these particles in a thickener. A pump 19 delivers this fraction through a conduit 20 to the sprayers 7. It will be understood that the non-magnetic particles contained in this fraction do not enter the separating device, so that they cannot unfavorably influence the separation. Part of these fine particles will be removed from the system since some will adhere to the fractions leaving at D, E and F. Furthermore, at a suitable spot, as for example, at the lowest point in the magnetic separator 15, indicated at H, an amount of rinsing water is drained so that the content of fine non-magnetizable particles in the circuit of the rinsing water cannot rise beyond the maximum admissible value. If desired, part of the non-magnetic fraction may also be supplied to the sprayers 12, through a conduit 21 by opening valve 21'.

The amount of water drained at H is controlled so that it Will be equal to the amount of water entering the system through the sprayers 12 and at G, minus the amount removed by the separated fractions at D, E and F. Losses of magnetite are made up by supplying fresh magnetite to the collecting tank 3, as at I.

In the embodiment according to Figure 2, the vibrating screen 11 has been replaced by a screening device 11' having a cylindrically-bent screening deck constructed in a manner similar to the device 9. The overflow material from screening device 9 is supplied tangentially to the bent screening deck, together with an amount of clean or clarified water supplied through a conduit 12'. The undersize material coming from screen 11', whose deck, like that of the screen comprises cross bars defining 0.4 min-Wide slots therebetween, contains about 90 percent of the liquid supplied to the screen and particles smaller than 0.2 mm., so that in collecting tank 13 another 9 percent of the magnetite originally present in the dilute suspension is caught. Otherwise, the diagram is identical with that shown in Figure 1.

In the embodiment according to Fig. 3, the overflow coming from screening device 9 is led to a second magnetic separator 22. Moreover, part of the dilute nonmagnetic fraction from the first magnetic separator may be added so that the material to be treated which is introduced into the magnetic separator 22 will be in the most suitable concentration. In the magnetic separator, a magnetite fraction therefrom is collected in a tank 23 and may be sent from there either directly to the separating device 1, to the collecting tank 3, or to the feed box of magnetic separator 15. A non-magnetic fraction containing the coarse and part of the fine non-magnetic particles is also recovered from the separator 22 and is collected in a tank 24. This latter fraction can either be drained at F or be thickened in a cyclone 25 by turning valve 25'. The clarified overflow from the cyclone is returned through a conduit 26 to the sprayers over the screens, and the thickened non-magnetizable particles are discharged at F".

In the diagrams described above various alterations may be made without essentially changing the invention.

For example, in Figure 3 the magnetic separator 22 may be placed after the screen 11', in order to catch magnetite left in the overflow coming from this screen. As a rule, this will not be necessary, however, as the overflow fraction from this screen contains only about 1 percent of the magnetite present in the rinsed-off suspension. However, in washeries where all magnetite is rinsed off, and consequently no draining screens 2 are applied, this arrangement may be advantageous.

It can thus be seen that there has been provided by the present invention a process in which the concentrated suspension returned to the separating device contains a minimum amount of non-magnetizable particles. Moreover, no thickener is needed for separating off the nonmagnetizable particles.

These advantageous results are obtained by the present invention since the dilute suspension recovered from the screens is first supplied substantially tangentially to the concave side of a cylindrically-bent screening surface where an undersize fraction is obtained which is substantially free of non-magnetic particles. magnetic fraction is then fed to the magnetic separator and the magnetic fraction separated ofi in this magnetic separator is returned to the separating device. Moreover,

This undersizethe dilute fraction from the magnetic separator con: tains so few non-magnetic particles that it can be fed di: rectly to the sprayers over the rinsing screens.

The coarse fraction retained on the cylindrically-bent screening surface will consist mainly of non-magnetizable particles, whereas the fraction passing through the screen apertures and supplied to the magnetic separator consists mainly of the fine magnetizable particles. The result is that only a small number of non-magnetizable particles are enclosed within the concentrated magnetic fraction separated off in the magnetic separator. Consequently, the separating medium in the separating device is considerably less contaminated than in the conventional process, so that the viscosity of this medium is lower and the separation is more accurate.

As most of the non-magnetizable particles are retained by the screen 9, the non-magnetic fraction coming from the magnetic separator contains only a small portion of these particles. Therefore, it is not necessary to thicken this fraction in order to remove the non-magnetizable particles and the fraction can be led directly to the sprayers over the rinsing screens. This means a considerable simplification of the separating apparatus over what has been considered necessary heretofore.

The application of the screening apparatus with bent screening deck, in which the size of the screen apertures measured in the direction in which the material to be treated moves, is equal to or smaller than the size of the same openings measured perpendicular to this direction of movement, as described in the aforesaid patent application, has the advantage that the cut is made at a particle size considerably smaller than the minimum dimension of these screen apertures. Therefore, a large capacity can be obtained with a comparatively small screening surface without danger of the screen apertures becoming blocked. The application of the conventional screening devices for separating the contaminations from the dilute suspension would be attended by very great difiiculties owing to the small capacity of these devices, the short life of the screening gauze and blockage of the screen apertures and therefore would be impractical.

The coarse fraction obtained in screening the dilute suspension on the screen 9 still contains magnetizable particles. These may be removed by subjecting this fraction to a second screening treatment. The resulting fine fraction, which contains the remaining magnetizable particles, is led to the magnetic separator for further purification. This secondary screening treatment is preferably carried out while water and/or part of the non-magnetic fraction separated off in the magnetic separator is being added. The screen for the secondary screening of the coarse fraction may be, like the primary screen, a screen with a cylindrically-bent screening deck, the fraction to be screened being supplied, if desired, together with an amount of water and/or part of the non-magnetic fraction from the magnetic separator, tangentially to this screening deck. However, as the coarse fraction constitutes only a small portion of the dilute suspension collected under the rinsing screens and a large portion of the particles that may cause obstruction have been included in the undersize product from the preceding screen, the screen for the secondary treatment of the coarse fraction may be a vibrating screen as indicated in Figure 1. In this case water and/or part of the non-magnetic fraction coming from the magnetic separator is supplied to Sprayers mounted over this screen, for rinsing the fine magnetizable particles.

Furthermore, it is possible, according to the invention, to treat the coarse fraction obtained in the screening of the rinsed suspension in a second magnetic separator, in order to recover the magnetizable particles still present in this fraction. The non-magnetic fraction discharged from this magnetic separator may then be supplied to a hydrocyclone to be dewatered, the resulting clarified liquid being led to the sprayers over the screens.

It thus will be seen that the objects of this invention have been fully and effectively accomplished. It will be realized, however, that the foregoing specific embodiment has been shown and described only for the purpose of illustrating the principles of this invention and is subject to extensive change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.

I claim:

1. Apparatus for separating mixtures of solid substances into fractions differing in specific gravity comprising a separating device for separating mixtures of solid substances into fractions difiering in specific gravity by means of a separating suspension comprising magnetizable particles suspended in water, screens for receiving the separated fractions, means for rinsing oh the suspension adhering to the separated fraction on said screens, a screening deck having an apertured cylindrically bent screening surface, means for supplying the suspension substantially tangentially to the concave side of said screening surface for separating an undersize fraction therefrom, the remainder of the suspension constituting the coarse fraction, means for treating the coarse fraction recovered from said screening deck to separate said coarse fraction into asecondary coarse fraction and a secondary fine fraction, a magnetic separator, means for supplying the undersize fraction from said screening deck and said secondary fine fraction to said magnetic separator, and means for supplying at least a part of the nonmagnetic fraction recovered from said magnetic separator to said rinsing means.

2. Apparatus as defined in claim 1 wherein said coarse fraction treating means comprises screen means.

3. Apparatus as defined in claim 2 including means forv supplying a part of the non-magnetic fraction recovered from the magnetic separator to said screen means.

4. Apparatus as defined in claim 2 wherein said screen means comprises a cylindrically-bent screening deck having screening apertures therein arranged to receive said coarse fraction in a direction substantially tangentially to its concave side.

5. Apparatus as defined in claim 2 wherein said screen means comprises a vibrating screen.

, 6. Apparatus for separating mixtures of solid substances into fractions differing in specific gravity comprising a separating device for separating mixtures of solid substances into fractions differing in specific gravity by means of a separating suspension comprising magnetizable particles suspended in water, screens for receiving the separated fractions, means for rinsing off the suspension adhering to the separated fraction on said screens, a screening deck having an apertured cylindrically bent screenin; surface, means for supplying the suspension substantially tangentially to the concave side of said screening surface for separating an undersize fraction therefrom, the remainder of the suspension constituting the coarse fraction, means for treating said coarse fraction including a magnetic separator, and means for supplying said coarse fraction to said magnetic separator, a second magnetic separator, means for supplying the undersize fraction from said screening deck to said second magnetic separator, and means for supplying at least a part of the nonmagnetic fraction recovered from said second magnetic separator to said rinsing means.

7. Apparatus as defined in claim 6 wherein said coarse fraction treating means further includes a hydrocyclone, means for supplying under pressure the non-magnetic fraction recovered from the magnetic separator tangentially to said hydrocyclone, and means for supplying at least a part of the dilute fraction recovered from said hydrocyclone to said rinsing means.

8. A process for separating mixtures of solid substances into fractions differing in specific gravity which comprises separating the mixture in a separator device with the use of a separating suspension comprising magnetizable particles suspended in water, passing the separated fractions recovered from the separating device on screens and spraying the same while on said screens so as to rinse oif the suspension adhering thereto, feeding a dilute suspension recovered from the screens along a cylindrically shaped path in layer formation, blocking small successive layers at spaced intervals along said path from one surface of the layer formation flow to separate an undersize fraction from the dilute suspension, the remaining dilute suspension at the end of said path constituting a coarse fraction, subjecting said coarse fraction to a secondary treatment to recover a secondary fine fraction, passing said undersize fraction and said secondary fine fraction through a magnetic separator, returning a concentrated fraction of magnetizable particles recovered from the magnetic separator to the separating device, and utilizing without subsequent treatment at least a part of the non magnetic fractions recovered from the magnetic separator as the spraying medium for the separated fractions on said screen.

9. A process as defined in claim 8 wherein said coarse fraction secondary treatment includes a screening treatment.

10. A process as defined in claim 21 wherein a part of the non-magnetic fraction recovered from the magnetic separator is used in carrying out the secondary screening treatment of the coarse fraction.

11. A process for separating mixtures of solid substances into fractions differing in specific gravity which comprises separating the mixture in a separating device with the use of a separating suspension comprising magnetizable particles suspended in water, passing the separated fractions recovered from the separating device on screens and spraying the same while on said screens so as to rinse off the suspension adhering thereto, feeding a dilute suspension recovered from the screens along a cylindrically shaped path in layer formation, blocking small successive layers at spaced intervals along said path from one surface of the layer formation flow to separate an undersize fraction from the dilute suspension, the remaining dilute suspension at the end of said path constituting a coarse fraction, treating said coarse fraction in a magnetic separator in order to recover the magnetizable particles left in this fraction, passing said undersize fraction through a second magnetic separator, returning a concentrated fraction of magnetizable particles recovered from the second magnetic separator to the separating device, and utilizing without subsequent treatment at least a part of the non-magnetic fraction recovered from the second magnetic separator as the spraying medium for the separated fractions on said screen.

12. A process as defined in claim 11 wherein the nonmagnetic fraction recovered from the second magnetic separator is fed into a hydrocyclone and at least a part of the dilute fraction recovered from the hydrocyclone is led to the screens and sprayed thereover.

References Cited in the file of this patent UNITED STATES PATENTS 2,690,261 Maust Sept. 28, 1954 2,692,048 Davis Oct. 19, 1954 2,872,041 Fontein Feb. 3, 1959 OTHER REFERENCES Quarterly of the Colorado School of Mines, Ianuary 1948, volume 43, number 1, page 31, Figure 10.

F. J. Fontein, Gluckuf, volume 91, number 27-28, Jul 2, 1955, pages 781-786 (Abstract). 

